Why does it have to be 1500W??. The power loading of the element doesn't really reflect the output power, only the initial heat up time

. For instance you can find 900W machines with a similar output to a 1500W, the only difference being that the 900W can possibly take 1 min longer to reach operating temperature than the 1500W!.
Once the heat exchanger reaches operating temp the wattage and heat up times are no longer relevant. Most modern machines (With the exception of Budget, Budget ones) maintain their operating temperature without losing production of smoke. Which means that even the temp drops slightly and the element is energised the smoke will still be produced

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Finding a Quiet Machine for £200 is going to be very difficult. Most machines sound like your windscreen washer pump on a car (Which on cheap models is exactly what is used to pump the fluid). Also the hissing produced by the fluid reacting with the element is hard to isolate especially at high power levels so you may have to consider an Aerosol fed machine, where the pressure is produced by the aerosol gas rather than an Electronic pump.
I am working on adding to my range of machines namely from Chauvet and EU. The DTS Models on the site will give you the level of smoke and meet your budget, but are noisy in operation.